We applied multiple geochemical tracers ( 87 Sr/ 86 Sr, [Sr], d 13 C, and d 18 O) to waters and carbonates of the lower Colorado River system to evaluate its paleohydrology over the past 12 Ma. Modern springs in Grand Canyon reflect mixing of deeply derived (endogenic) fluids with meteoric (epigenic) recharge. Travertine (<1 Ma) and speleothems (2-4 Ma) yield 87 Sr/ 86 Sr and d 13 C and d 18 O values that overlap with associated water values, providing justification for use of carbonates as a proxy for the waters from which they were deposited. The Hualapai Limestone (12-6 Ma) and Bouse Formation (5.6-4.8 Ma) record paleohydrology immediately prior to and during integration of the Colorado River. The Hualapai Limestone was deposited from 12 Ma (new ash age) to 6 Ma; carbonates thicken eastward to ~210 m toward the Grand Wash fault, suggesting that deposition was synchronous with fault slip. A fanningdip geometry is suggested by correlation of ashes between subbasins using tephrochronology. New detrital-zircon ages are consistent with the "Muddy Creek constraint," which posits that Grand Wash Trough was internally drained prior to 6 Ma, with limited or no Colorado Plateau detritus, and that Grand Wash basin was sedimentologically distinct from Gregg and Temple basins until after 6 Ma. New isotopic data from Hualapai Limestone of Grand Wash basin show values and ranges of 87 Sr/ 86 Sr, d 13 C, and d 18 O that are similar to Grand Canyon springs and travertines, suggesting a long-lived springfed lake/marsh system sourced from western Colorado Plateau groundwater. Progressive up-section decrease in 87 Sr/ 86 Sr and d 13 C and increase in d 18 O in the uppermost 50 m of the Hualapai Limestone indicate an increase in meteoric water relative to endogenic inputs, which we interpret to record progressively increased input of high-elevation Colorado Plateau groundwater from ca. 8 to 6 Ma. Grand Wash, Hualapai, Gregg, and Temple basins, although potentially connected by groundwater, were hydrochemically distinct basins before ca. 6 Ma. The 87 Sr/ 86 Sr, d 13 C, and d 18 O chemostratigraphic trends are compatible with a model for downward integration of Hualapai basins by groundwater sapping and lake spillover.The Bouse Limestone (5.6-4.8 Ma) was also deposited in several hydrochemically distinct basins separated by bedrock divides. Northern Bouse basins (Cottonwood, Mojave, Havasu) have carbonate chemistry that is nonmarine. The 87 Sr/ 86 Sr data suggest that water in these basins was derived from mixing of high-87 Sr/ 86 Sr Lake Hualapai waters with lower-87 Sr/ 86 Sr, first-arriving "Colorado River" waters. Covariation trends of d 13 C and d 18 O suggest that newly integrated Grand Wash, Gregg, and Temple basin waters were integrated downward to the Cottonwood and Mojave basins at ca. 5-6 Ma. Southern, potentially younger Bouse basins are distinct hydrochemically from each other, which suggests incomplete mixing during continued downward integration of internally drained basins. Bouse carbonates display a southward trend ...

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